1. Field of the Invention
This invention relates to a method for stabilizing metallic liquid mercury in an environmentally benign, monolithic solid form for storage or disposal. The method of this invention is capable of producing an amalgam of copper and mercury in which the mercury comprises greater than about 50% by weight of the total weight of the amalgam. The amalgam produced in accordance with the method of this invention is resistant to deformation, breakage, chemical corrosion, and environmental release of its mercury content. The mercury thus sequestered can be disposed of by burial, for example, or it can be separated from the copper and recovered by distillation after storage. The method of this invention does not require the use of silver, tin, nickel, indium or other expensive metals or alloys and, thus, it is quite economical.
2. Description of Prior Art
An amalgam is an alloy of mercury and at least one other metal. Many, but not all, metals form alloys with mercury. Most of the amalgams are solid solutions of variable proportions. For example, it is well known that gold readily absorbs mercury to form either solid amalgam or, with higher mercury content, a liquid metal solution. This absorption of mercury by gold is the basis for a variety of methods used to remove harmful mercury vapor from gases and liquids and it can also be used to measure the mercury content in fluids. U.S. Pat. No. 5,409,522 teaches a method and apparatus for removing particulate material and mercury-containing compounds from a gas stream which includes a mercury collection means for removing the mercury-containing compounds. The mercury collection means include a mercury collection surface having a mercury adsorptive capacity no less than about 0.1 micrograms/cm.sup.2. Suitable mercury collection surfaces are indicated to include gold, silver, sodium, palladium, potassium, platinum, copper, and mixtures thereof, with the most preferred collection surface being gold. The absorption of mercury by gold also provides a method for extracting gold from it ores. No pretreatment of the gold surface is required, and the mercury can be vaporized from the gold and separated by heating. However, it will be apparent that as a disposable sorbent, gold is too expensive.
It has also long been known that mercury forms a stable solid amalgam with powdered silver at room temperature. Alloy powders containing minor amounts of copper, zinc, tin, and/or indium with silver in the dominant proportion behave similarly to pure silver powder in this regard. See, for example, U.S. Pat. No. 4,164,419 which teaches a powdered alloy for dental amalgam consisting essentially, by weight, of at least 50% silver, 20-30% tin, 3-6% indium and 11-20% copper. When mercury is first mixed with silver-rich powders, it forms a metallic slurry having the consistency of a paste which can be formed and shaped by hand. The paste solidifies in minutes and reaches maximum strength in roughly 24 hours. This is the dental amalgam used to fill cavities in teeth. It is hard enough to withstand the pressures of chewing, often for decades. And, although dissolved mercury is quite toxic, this amalgam is so inert to chemical reaction that no harm is done even considering the broad pH range of foods, from about 1.8 to about 8.5, to which the amalgam is exposed. Even with this broad pH range, combined with chewing abrasion, mercury is not released from fillings into the saliva in harmful amounts. However, as in the case of gold, silver is relatively expensive for use as a disposable sorbent. Accordingly, it is apparent that there is a need for an amalgam having the hardness and chemical stability of dental amalgam that does not require silver or any other expensive elements. Such an alloy would be useful not only for the storage and disposal of mercury, but also in room temperature casting, joining and repair of metal objects.
U.S. Pat. No. 5,034,054 teaches a method for treating mercury in preparation for disposal in which the mercury is mixed with an inorganic powder such as copper, zinc, nickel, or sulfur in a ratio of powder to mercury of at least 1:1 and preferably more on the order of 3:1 resulting in an amalgam in solid form in which the mercury is permanently bonded to the powder. To promote formation of the amalgam, the '054 patent teaches the use of mechanical "non-intrusive compound agitation" to overcome surface oxidation and bring the metals together at an atomic scale. The resulting amalgamation resembles a powderous copper. Attempts to operate the process at powder to mercury ratios of less then 1:1 result in a portion of the liquid mercury not being absorbed by the powder. Accordingly, it is apparent that, in order to reduce the waste volume and sorbent requirements for amalgamation, an amalgamation process which is capable of producing amalgams in which the mercury content is greater than 50% of the total mass is highly desirable.
As previously stated, the product of the process of the '054 patent is a "powderous" amalgam, which would be expected for a mixture containing less mercury than sorbent. Leaching of a toxic substance into ground water is approximately proportional to the specific surface area (cm.sup.2 /g) of the toxic substance exposed to the water. For example, the specific surface area of a 20-micron powder is one billion times the specific surface area of a 2 cm monolithic casting of the same shape. As a result, the leaching to be expected from such a casting is one billionth the leaching of the powder under equivalent conditions. Accordingly, it will be apparent that for applications in which the amalgam is to be buried, such as in land fills, a monolithic amalgam is preferable over powderous amalgams of similar composition.
In addition to copper, U.S. Pat. No. 5,034,054 also teaches that zinc, because it readily forms a solid amalgam, is a suitable medium in which to sequester mercury. See also U.S. Pat. No. 4,076,553, U.S. Pat. No. 3,975,298, and Japanese Patent Publication 2,296,926. However, because zinc has an electrode potential of 0.7628 volts, which is well above that of hydrogen, it is a very reactive metal. By comparison, copper has a potential of -0.345 volts, which is below that of hydrogen and is consequently much less reactive. In addition, zinc amalgam is attacked by water resulting in the formation of hydrogen and zinc hydroxide in contrast to copper amalgam which remains stable in water. Given sufficient water, all the zinc metal is consumed and the amalgamated mercury returns to its free and hazardous liquid form. Accordingly, it will be apparent that reactivity is unsuitable in a sorbent for storage or burial, rendering zinc as unsuitable for mercury sequestration where the resulting solid amalgam is to be buried or stored.